UNSUPPORTED_CLIENT Callout Error in Salesforce

Error:

[{"message":"HTTPS Required","errorCode":"UNSUPPORTED_CLIENT"}]

Solution:

Unsupported_Client error occurs when making callout from salesforce using http web server instead of https. There are two possible solutions to fix this issue,

Option 1:

You can change the security setting in your Salesforce account to allow API communication over HTTP as follows:

1. Log into Salesforce.com.

2. Click Setup, click Security Controls, and then click Session Settings.

3. Uncheck the option labeled Require secure connections (HTTPS).

Note: To change to HTTP, you need to contact salesforce.com

Option 2:

1. HttpRequest req = new HttpRequest();
2. req.setHeader('Authorization', 'Bearer ' + UserInfo.getSessionID());
3. req.setHeader('Content-Type', 'application/json');
4. req.setEndpoint('https://na15.salesforce.com/services/data/v36.0/tooling/sobjects/');
5. req.setMethod('GET');

End point address should be https.